Monday, October 14, 2013

Checking your Compass with the Sun

We have just added this new section to our training materials, which we post here for feedback and comments.  Earlier sections of the text addressed compass checks by simply swinging ship promptly and watching that the shadow bearing does not change. Here we expand the use of the sun with a more versatile and accurate method.

The sun method just described uses several approximations and also calls for changing course to make the checks. Often we are underway and just need to be sure the compass is right on our present heading, and then we will worry about other headings when we can. This comes up, for example, in an ocean yacht race, where some doubt about the steering compass might arise because the electronic heading sensors are reporting other headings; in a race you do not want to stop and swing the ship.

But we do not have to. Here is a quick and easy solution that you can use in the ocean or in your local lake. We are relying now on celestial navigation in a sense, but we do not need any of the details if we are on land and have access to the Internet.  We just assume if you are in the ocean, you have the full tools to solve this with conventional cel nav methods.

We start with the measurement of Figure 12.9-1 where we learned that the compass bearing to the sun was 135 C when the vessel was headed 000 C. Note that this heading does not matter for this application. We just happen to have a nice picture with the compass in that direction. For this method, you measure the sun bearing on whatever heading you happen to be on.

Figure 12.9-1. Reading the sun’s bearing form the reciprocal of the shadow pin bearing. Here the shadow is at 315 C, so the sun’s bearing at the moment is 135 C, when the vessel was headed 000 C. From convenient resources on line we can determine the true bearing of the sun at this time, and from this and the known variation for our location we compute the magnetic bearing of the sun. The difference is then the deviation of the compass on our present heading.

But we do now need more information. We need to know the time accurate to within a minute or so, and we need to know our location. Both you can get from the GPS. Now we need to look up or compute what the true bearing of the sun was at this moment based on cel nav principles, or we just go online and look it up.

Go the for a quick link to the right place at the US Naval Observatory. Then type in the time of the bearing and your Lat-Lon. As example is in Figure 12.9-2, which assumes we were headed north in Chesapeake Bay on Oct 14, 2013, and we recorded the bearing at 0930 EDT, which is 1330 UTC.  The true bearing to the sun at the time was 123.5º.

Figure 12.9-2. Data from the USNO. The true bearing of the sun (Zn) is 123.5º at 0930 EDT from this location in Chesapeake Bay, VA. We also see that the height of the sun at this time (called Hc) was 24º 29.5’ above the horizon, along with other data we are not using. We can convert this true bearing to a magnetic bearing using the local magnetic variation

Next we need the local magnetic variation which we could get from a chart, but for this type of precise compass check we might want to go back online and get the most accurate and up to date value. This you find at the National Geodetic Center ( for specific times and locations. For our example time and location the correct value is 11º 05.3’ W, or about 11.1º W.   
Input screen for online GeoMag. Note you can download a Windows version of GeoMag for your own computer if you like. It makes a nice backup underway. Note too, land navigators call variation "declination," but this is never done in marine navigation, because we have other meanings for that name in marine navigation.

Output screen from online GeoMag

So the proper magnetic bearing of the sun is 123.5 + 11.1 = 134.6 M. The compass showed 135 C, so the deviation on heading 000 C is -0.4º which would be called  0.4º W.

We  have learned that this compass is essentially correct at this heading, as it is difficult to be confident we have read the shadow bearing to this precision. But the main point is, this method has no other approximations in it. The result you get is as accurate as you can read the shadow bearing on the compass card.

This is a very powerful method. It would be instructive to try it once or twice whenever you see a nice shadow on your compass card, no matter where you are.  Just write down the shadow bearing, the time, and the location, and your actual compass heading at the time. Then you can check the compass for that heading when you get back home. If convenient, record the data headed roughly north or south by compass and also roughly east or west by compass, and then you will have a pretty good analysis of your compass with a few minutes of paperwork at home.


The Unlikely Boatbuilder said...

I can’t believe that no one has asked this question, but what of the heading wasn’t exactly 0 magnetic ? I guess you have to subtract the course from the magnetic bearing to the sun?

The Unlikely Boatbuilder said...

Or, hmmm... does the ship’s course even matter, since the compass card won’t change? I.e. it doesn’t matter if the boat rotates under the compass card. Interesting...

David Burch said...

Sorry for the long delay. This must have come in before we learned how to turn on notices. The ship's heading does matter because you are checking the compass only on that specific heading, but that check is done the same way for all headings.